Electronic device and electronic apparatus

ABSTRACT

An EL display having high operating performance and reliability is provided. LDD regions  15   a  through  15   d  of a switching TFT  201  formed in a pixel are formed such that they do not overlap gate electrodes  19   a  and  19   b  to provide a structure which is primarily intended for the reduction of an off-current. An LDD region  22  of a current control TFT  202  is formed such that it partially overlaps a gate electrode  35  to provide a structure which is primarily intended for the prevention of hot carrier injection and the reduction of an off-current. Appropriate TFT structures are thus provided depending on required functions to improve operational performance and reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an electronic device formed byfabricating a semiconductor element (element utilizing a semiconductorthin film) on a substrate and an electronic apparatus utilizing such anelectronic device as a display portion. Particularly, the presentinvention is a technique which is advantageously embodied in an activematrix EL (electroluminescence) display which is an electronic device.

2. Description of the Related Art

Recently, great advances have been made in techniques for forming TFTson a substrate, and application of the same to the development of activematrix displays is in progress. Especially, TFTs utilizing polysiliconfilms are capable of operations at a high speed because they have higherfield effect mobility compared to conventional TFTs utilizing amorphoussilicon films. This has made it possible to control pixels with adriving circuit formed on the same substrate (insulator) on which thepixels are formed unlike the prior art in which pixels have beencontrolled by a driving circuit outside the substrate.

Such active matrix displays are presently attracting attentions forvarious advantages including compactness of the displays, improved yieldand reduced throughput attributable to the fact that various circuitsand elements are fabricated on the same substrate.

Various circuits and element portions having various functions areformed on the substrate of an active matrix display. Therefore, whenelements are formed of TFTs, the TFTs are required to have differentperformance depending on the respective circuits and elements. Forexample, TFTs operating at a high speed are required for shift registersfor generating a timing signal and the like, and TFTs having asufficiently low off-current (a drain current that flows when a TFT isoff) are required for switching elements for accumulating electricalcharges.

In such a case, it is difficult to maintain performance requirements ofall circuits or elements only with TFTs having the same structure, whichcan be a serious obstacle to efforts toward improved performance ofactive matrix displays.

It is an object of the invention to provide an active matrix typeelectronic device having a pixel portion and driving circuit portionsprovided on the same insulator, in which TFTs having appropriatestructures are used depending on performance required for circuits orelements formed by the TFTs to provide high operating performance andreliability.

It is another object of the invention to improve the quality of imageson an electronic device (particularly, an active matrix type ELdisplay), thereby improving the quality of an electronic apparatusutilizing the same as a display portion.

SUMMARY OF THE INVENTION

In order to achieve the above-described objects, a principle of thepresent invention is that TFTs having optimum structures are allocatedto each pixel of an EL display taking in view of the elements includedin the pixel. That is, TFTs having different structures are present inthe same pixel.

Specifically, TFT structures oriented toward lower off-currents ratherthan higher operating speeds are preferable for elements for which asufficiently low off-current is the most important requirement(switching elements and the like). For elements through which a highcurrent must flow as the top priority, it is preferable to use TFTstructures oriented toward flow of high currents and suppression of verymuch problematic deterioration attributable to injection of hot carriersrather than the reduction of the off-current.

The present invention makes it possible to improve operating performanceand reliability of an EL display by using different TFTs appropriatelyon the same insulator. The principle of the present invention ischaracterized in that TFT structures are optimized not only in a pixelportion but also in driving circuit portions for driving the pixelportion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a sectional structure of a pixel portion of an EL display.

FIGS. 2A and 2B show a top surface structure of the pixel portion of theEL display.

FIGS. 3A through 3E show steps for fabricating an active matrix ELdisplay.

FIGS. 4A through 4D show steps for fabricating an active matrix ELdisplay.

FIGS. 5A through 5C show steps for fabricating an active matrix ELdisplay.

FIGS. 6A and 6B show sectional structures of a pixel portion of an ELdisplay.

FIG. 7 shows a configuration of elements in a pixel portion of an ELdisplay.

FIG. 8 shows a configuration of elements in a sampling circuit of an ELdisplay.

FIG. 9 shows a sectional structure of a pixel portion of an EL display.

FIGS. 10A and 10B shows a sectional structure of a pixel portion of anEL display.

FIGS. 11A and 11B show a top surface structure and a sectional structureof an EL display.

FIGS. 12A through 12C show circuit configurations of a pixel portion ofan EL display.

FIGS. 13A and 13B show circuit configurations of a pixel portion of anEL display.

FIGS. 14A and 14B show circuit configurations of a pixel portion of anEL display.

FIGS. 15A through 15F show specific examples of electronic apparatuses.

FIGS. 16A and 16B show specific examples of electronic apparatuses.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

A preferred embodiment of the invention will now be described withreference to FIGS. 1 and 2. FIG. 1 is a sectional view of a pixel of anEL display according to the invention. FIG. 2A is a plan view of thesame, and FIG. 2B shows a circuit configuration of the same. Inpractice, a plurality of such pixels are arranged in the form of amatrix to form a pixel portion (image display portion).

The sectional view of FIG. 1 shows a section along the line A-A′ in theplan view of FIG. 2A. Since common reference numbers are used in FIGS.1, 2A and 2B, those figures may be cross-referred appropriately. Whilethe plan view in FIG. 2A shows two pixels, they have the same structure.

In FIG. 1, 11 represents a substrate, and 12 represents an underlyingfilm (insulator). The substrate 11 may be a glass substrate,glass-ceramics substrate, quartz substrate, silicon substrate, ceramicssubstrate, metal substrate or plastic substrate (including a plasticfilm).

While the underlying film 12 is advantageous especially when a substrateincluding mobile ions or a conductive substrate is used, it may beomitted for a quartz substrate. An insulation film including silicon maybe provided as the underlying film 12. In the present specification, theterm “insulation film including silicon” specifically implies aninsulation film such as silicon oxide film, silicon nitride film orsilicon oxinitride film (expressed by SiO_(x)N_(y)) which includessilicon and oxygen or nitrogen in a predetermined ratio.

Here, two TFTs are formed in a pixel. 201 represents a TFT which servesas a switching element (hereinafter referred to as “switching circuit”),and 202 represents a TFT which controls the amount of a current flowingthrough an EL element (hereinafter referred to as “current controlTFT”). They are both n-channel type TFTs.

The switching TFT 201 is formed with an active layer including a sourceregion 13, a drain region 14, LDD regions 15 a through 15 d, a highdensity impurity region 16 and channel forming regions 17 a and 17 b, agate insulation film 18, gate electrodes 19 a and 19 b, a first layerinsulation film 20, a source line 21 and a drain line 22. As shown inFIG. 2A, the gate electrodes 19 a and 19 b are in a double gatestructure in which they branch from the same gate line 211.

The active layer is constituted by a semiconductor film having acrystalline structure. That is, it may be a monocrystallinesemiconductor film, polycrystalline semiconductor film ormicrocrystalline semiconductor film. The gate insulation film 18 may beconstituted by an insulation film including silicon. Any conductive filmmay be used for the gate electrodes, source line and drain line.

A storage capacitor 203 is connected to the switching TFT 201 (see FIG.2B). The storage capacitor 203 is formed by a capacitor formingsemiconductor region 23 electrically connected to the drain region 14,the gate insulation film 18 (which serves as a dielectric body forforming a capacitor where the storage capacitor 203 is formed) and acapacitor forming electrode 24. A connection line 25 is a line forapplying a fixed potential (a ground potential in this case) which isformed simultaneously with the source line 21 and drain line 22 andwhich is connected to a current supply line 212.

At this time, the LDD regions 15 a through 15 d of the switching 201 areprovided such that they will not overlap the gate electrodes 19 a and 19b with the gate insulation film 18 interposed.

The switching TFT 201 accumulates an electrical charge associated with avideo signal (a signal including image information) in the storagecapacitor when it is selected. Since the electrical charge must becontinually maintained in an unselected state, the leakage of theelectrical charge attributable to an off-current must be minimized. Inthis sense, the reduction of the off-current must be the top priority indesigning the switching TFT 201.

In order to reduce the off-current, it is further preferable to providean offset region (which is constituted by a semiconductor layer havingthe same composition as that of the channel forming region and to whichthe gate voltage is not applied) between the channel forming region andthe LDD region. In the case of a multi-gate structure having two or moregate electrodes, the high density impurity region provided between thechannel forming regions is effective in reducing the off-current.Although a multi-gate structure as in the present embodiment isdesirable, a single gate structure may be employed.

The current control TFT 202 is formed with an active layer including asource region 31, a drain region 32, an LDD region 33 and a channelforming region 34, a gate electrode 35, a first layer insulation film20, a source line 36 and a drain line 37. While the gate electrode 35has a single gate structure, a multi-gate structure may be employed.

As shown in FIG. 2A, the gate electrode 35 is electrically connected tothe drain region 14 of the switching TFT 201 through the drain line(which may be also referred to as “connection line”) 22. The source line36 is integral with the connection line 25 and is connected to thecurrent supply line 212 similarly to the same.

The current control TFT 202 is characterized in that the LDD region 33is provided between the drain region 32 and the channel forming region34 and in that the LDD region 33 has an area which overlaps the gateelectrode 35 with the gate insulation film 18 interposed and an areawhich does not overlap the same.

The current control TFT 202 supplies a current to cause the EL element204 to emit light and enables gray scale display by controlling theamount of the same. It is therefore necessary to take a countermeasureto deterioration attributable to the injection of hot carriers in orderto prevent deterioration even when a high current flows. The currentcontrol TFT 202 is kept in an off state to display black and, at thistime, a high off-current disables clear display of black to reducecontrast. It is therefore also necessary to suppress the off-current.

Referring to deterioration attributable to the injection of hotcarriers, structures in which the LDD region overlaps the gate electrodeare known to be very effective in preventing the same. However, sincethe off-current is increased if the entire LDD region overlaps the gateelectrode, the inventors have provided measures to deal with hotcarriers and an off-current by employing a novel structure in which anLDD region having an area which does not overlap a gate electrode.

The length of the area of the LDD region that overlaps the gateelectrode may be in the range from 1 to 3 μm (preferably from 0.3 to 1.5μm). An increase in a parasitic capacitance occurs when this length istoo large, and the effect of preventing hot carriers is reduced when itis too small. The length of the area of the LDD region that does notoverlap the gate electrode may be in the range from 1.0 to 3.5 μm(preferably from 1.5 to 2.0 μm). A sufficient flow of current cannot beachieved this length is too large, and the effect of reducing theoff-current is reduced when it is too small.

Since a parasitic capacitance is formed in the area of where the gateelectrode and LDD region overlap in the above-described structure, it ispreferable not to provide the same area between the source region 31 andchannel forming region 34. Since carriers (electrons in this case) flowsthrough the current control TFT always in the same direction, asufficient effect can be achieved by providing the LDD region only onthe side of the drain region.

As described above, two kinds of TFT having different structures areprovided in a pixel depending on the function of the same. In theillustrated example, both of the switching TFT 201 and current controlTFT 202 are n-channel type TFTs. This is very much advantageous inincreasing an effective emitting area of an EL element because ann-channel type TFT can be formed smaller than a p-channel type TFT.

P-channel type TFTs are advantageous in that they are substantially freefrom the problem attributable to hot carrier injection and in that theyhave a low off-current, and reports have already been made on examplesof the use of them as switching TFTs and current control TFTs. However,the present invention is further characterized in that a structure inwhich LDD regions are provided in different positions to solve theproblem attributable to hot carrier injection and the problem of theoff-current and in that all TFTs in all pixels can therefore ben-channel type TFTs.

41 represents a passivation film which is a silicon nitride film or asilicon oxinitride film. 42 represents a color filter, and 43 representsa fluorescent body (also referred to as “fluorescent dye layer”). Bothof them have the same combination of colors and include red (R), green(G) and blue (B) dyes. The color filter 42 is provided to improve colorpurity, and the fluorescent body 43 is provided to perform colorconversion.

There are four general types of methods for color representation on ELdisplays, i.e., a method wherein three types of EL elements associatedwith R, G and B are formed, a method wherein EL elements emitting whitelight are combined with a color filter, a method wherein EL elementsemitting blue light are combined with a fluorescent body (fluorescentcolor conversion layer: CCM) and a method wherein a transparentelectrode is used as a cathode (counter electrode) and EL elementsassociated R, G and B are overlapped therewith.

The structure shown in FIG. 1 is an example of the method wherein ELelements emitting blue light are combined with a fluorescent body. Alight emitting layer emitting blue light is used as the EL element 204to generate light having a wavelength in the blue range includingultraviolet light, and the fluorescent body 43 is excited by the lightto generate light in red, green or blue. The color purity of the lightis improved by the color filter 42 which then outputs the light.

The present invention may be carried out regardless of the lightemitting method used, and all of the above-described methods maytherefore be used in the present invention.

After the color filter 42 and fluorescent body 43 are formed,planarization is carried out on the second layer insulation film 44. Anorganic resin film is preferably used as the second layer insulationfilm 44, and polyimide, acrylic resin or BCB (benzocyclobutene) may beused. Obviously, an inorganic film may be used if it can be sufficientlyplanarized.

45 represents a pixel electrode (anode of the EL element) which isconstituted by a transparent conductive film and which is connected tothe drain line 37 of the current control TFT by providing a contact holein the second layer insulation film 44 and passivation film 41.

An EL layer (which is preferably made of an organic material) 46, acathode 47 and a protective electrode 48 are sequentially formed on thepixel electrode 45. A multi-layer structure is often used for the ELlayer 46, although it may have either of single-layer and multi-layerstructures. While various multi-layer structures for EL layers have beenproposed which are combinations of an electron transport layer and ahole transport layer in addition to a light emitting layer, the presentinvention accommodates any of such methods.

A material including magnesium (Mg), lithium or calcium (Ca) having asmall work function is used for the cathode 47. A MgAg electrode ispreferably used. The protective electrode 48 is an electrode provided toprotecting the cathode 47 from ambient moisture which is formed using amaterial including aluminum (Al) or silver (Ag).

The EL layer 46 and cathode 47 are preferably continuously formedwithout exposing them to the atmosphere. That is, the EL layer andcathode are preferably continuously formed regardless of how they arestacked. The purpose is to prevent the EL layer from absorbing moistureas a result of exposure to the atmosphere when an organic material isused which is very much vulnerable to moisture. It is further preferableto continuously form not only the EL layer 46 and cathode 47 but alsothe protective electrode 48 thereon.

The EL display according to the invention has a pixel portion formed bypixels having a structure as described above, and TFTs having differentstructures are provided in each pixel depending on the function thereof.This makes it possible to form a switching TFT having a sufficiently lowoff-current and a current control TFT resistant to hot carrier injectionin the same pixel, thereby allowing the formation of an EL display whichhas high reliability and which is capable of preferable display ofimages.

The present invention is not limited to a pixel portion of an EL displayand can be equally applied to driving circuit portions of an activematrix EL display in which the driving circuit portions and a pixelportion are formed on the same substrate. Specifically, one principle ofthe invention is to provide TFTs having different structures in eitherof a circuit driving portion and a pixel portion depending on thefunctions required by the circuits or elements.

The present invention can be applied also to the formation of signalprocessing circuits in addition to driving circuit portions and pixelportions as described above. Such signal processing circuits includesignal dividing circuits, D-A converters, γ-correction circuits,boosting circuits and differential amplifier circuits.

A more detailed description will be made on the present invention havingthe above-described configuration with reference to preferredembodiments.

Embodiment 1

A first embodiment of the invention will be described with reference toFIGS. 3A through 5C. A description will be made here on a method forfabricating TFTs of a pixel portion and driving circuit portionsprovided around the same simultaneously. For simplicity of thedescription, only a CMOS circuit is shown which is a basic circuit forsuch driving circuits.

First, as shown in FIG. 3A, an underlying film 301 having a thickness of300 nm is formed on a glass substrate 300. In the present embodiment,the a silicon oxinitride film is used as the underlying film 301. Atthis time, the density of nitrogen in the region in contact with theglass substrate 300 is preferably in the range from 10 to 25 wt %.

Next, an amorphous silicon film (not shown) having a thickness of 50 nmis formed on the underlying film 301 using a known film forming method.The film is not limited to an amorphous silicon film, and it may be anysemiconductor film (and any microcrystalline semiconductor film)including an amorphous structure. The film may alternatively be acompound semiconductor film including an amorphous structure such as anamorphous silicon germanium film. The thickness may be in the range from20 to 100 nm.

The amorphous silicon film is then crystallized using known techniquesto form a crystalline silicon film (also referred to “polycrystallinesilicon film” or “polysilicon film”) 302. Known methods forcrystallization include thermal crystallization utilizing anelectrically heated furnace, laser anneal crystallization utilizinglaser light and lamp anneal crystallization utilizing infrared light. Inthe present embodiment, crystallization is performed using excimer laserlight utilizing XeCl gas.

While pulse-oscillated excimer laser light formed in a linearconfiguration is used in the present embodiment, a rectangularconfiguration may alternatively be used. Continuously oscillated argonlaser light or continuously oscillated excimer laser light may be used.

Then, as shown in FIG. 3B, a protective film 303 constituted by asilicon oxide film is formed to a thickness of 130 nm on the crystallinesilicon film 302. A thickness within the range from 100 to 200 nm(preferably from 130 to 170 nm) may be chosen. Other types of insulationfilms may be used as long as silicon is included therein. The protectivefilm 303 is provided to prevent direct exposure of the crystallinesilicon film to plasma during doping with an impurity and to enabledelicate density control.

Resist masks 304 a through 304 c are formed on the protective film toallow doping with an impurity element that provides n-type conductivity(hereinafter referred to as “n-type impurity element”) through theprotective film 303. As the n-type impurity element, an elementbelonging to the group V, typically, phosphorus or arsenic may be used.In the present embodiment, phosphorus is added in a density of 1×10¹⁸atoms/cm³ using a plasma doping process in which phosphine (PH₃) isplasma-excited without performing mass separation on the same. It isobviously possible to use an ion implantation process which involvesmass separation.

The dose is adjusted such that n-type impurity regions 305 through 307formed at this step include the n-type impurity element in a density inthe range from 2×10¹⁶ to 5×10¹⁹ atoms/cm³ (typically, from 5×10¹⁷ to5×10¹⁸ atoms/cm³). The n-type impurity region 306 corresponds to thecapacitor forming semiconductor region 23 shown in FIG. 1.

Next, as shown in FIG. 3C, the protective film 303 is removed toactivate the added element belonging to the group V. While any knowntechnique may be used as means for activation, activation is carried outby means of illumination with excimer laser light. Obviously, theinvention is not limited to excimer laser light, and pulse-oscillated orcontinuously oscillated laser light may be used. Since the purpose is toactivate the added impurity element, illumination is preferably carriedout with an energy at which the crystalline silicon film is not melted.The illumination with laser light may be carried out with the protectivefilm 303 unremoved.

When the impurity element is illuminated with laser light, activationmay be simultaneously performed using furnace annealing or lampannealing. Referring to activation using furnace annealing, a thermalprocess at a temperature in the range from 450 to 550° C. can be carriedout taking the heat-resistance of the substrate into consideration. Theactivation may be carried out using only furnace annealing or lampannealing.

As a result of this step, the edges of the n-type impurity regions 305through 307, i.e., the boundaries (bonding portions) between the n-typeimpurity regions 305 through 307 and the regions around the same whichare not doped with the n-type impurity element becomes clear. Therefore,very preferable bonding portions can be formed between the LDD regionsand the channel forming region when the TFT is completed later.

Next, as shown in FIG. 3D, unnecessary portions of the crystallinesilicon film are removed to form island-shaped semiconductor films(hereinafter referred to as “active layers”) 308 through 311.

Next, as shown in FIG. 3E, a gate insulation film 312 is formed to coverthe active layers 308 through 311. An insulation film including siliconwith a thickness in the range from 10 to 200 nm (preferably in the rangefrom 50 to 150 nm) may be used as the gate insulation film 312. Thisfilm may have either of single-layer or multi-layer structures. In thepresent embodiment, a 110 nm thick silicon oxinitride film is used.

Next, a conductive film having a thickness in the range from 200 to 400nm is formed and patterned to form gate electrodes 313 through 317 and acapacitor forming electrode 318. While a gate electrode and a gate linemay be described as separate elements in this specification, the gateelectrode may be regarded as being included in the gate line because theportion to serve as an electrode is called “gate electrode” only forconvenience. This equally applies to the capacitor forming electrode,and the portion of the same which is not serving as an electrode may bereferred to “capacitor forming line”.

While the gate electrode may be constituted by single-layer conductivefilms, multi-layer films such as double-layer or triple-layer structuresare preferably used as needed. Any known conductive film may be used asthe material for the gate electrodes.

Specifically, it is possible to use thin films including tantalum (Ta),titanium (Ti), molybdenum (Mo), tungsten (W), chromium (Cr) orconductive silicon (Si) or thin films which are nitrides of the same(typically tantalum nitride films, tungsten nitride films or titaniumnitride films) or alloy films which are combinations of the aboveelements (typically Mo—W alloys or Mo—Ta alloys) or silicide filmsincluding the above elements (typically tungsten silicide films ortitanium silicide films). Such films may be used in either ofsingle-layer and multi-layer structures.

In the present embodiment, multi-layer films formed by a 50 nm thicktantalum nitride (TaN) film and 350 nm thick Ta film are used. They maybe formed using a sputtering process.

An inert gas such as Xe, Ne or the like may be used as the sputteringgas to prevent the films from coming off due to stress.

At this time, the gate electrodes 314 and 317 are formed such that theyoverlap a part of the n-type impurity regions 305 and 307 respectivelywith the gate insulation film 312 interposed. Such overlaps become LDDregions which overlap the gate electrodes later. While the gateelectrodes 315 and 316 look like two separate elements in the section,in practice, they are constituted by a single continuous pattern.

A capacitor forming electrode 318 is formed on the n-type impurityregion 306 with the gate insulation film 312 interposed. At this time,the insulation film provided as the gate insulation film 312 is usedhere as a dielectric body for a storage capacitor to form a storagecapacitor constituted by the n-type impurity region (capacitor formingsemiconductor region) 306, gate insulation film 312 and capacitorforming electrode 318.

Next, as shown in FIG. 4A, an n-type impurity element (which isphosphorus in the present embodiment) is added in a self-aligning mannerusing the gate electrodes 313 through 317 and capacitor formingelectrode 318 as masks. An adjustment is performed such that resultantimpurity regions 319 through 325 are doped with phosphorus in a densityin the range from ½ to 1/10 (typically from ⅓ to ¼) of that in then-type impurity regions 305 through 307. Specifically, a density in therange from 1×10¹⁶ to 5×10¹⁸ atoms/cm³ (typically from 3×10¹⁷ to 3×10¹⁸atoms/cm³ is preferable.

Next, as shown in FIG. 4B, resist masks 326 a through 326 c are formedto cover the gate electrodes and the like, and an n-type impurityelement (which is phosphorus in the present embodiment) is added to formimpurity regions 327 through 334 heavily doped with phosphorus. An iondoping process utilizing phosphine (PH₃) is performed again, and thedensity of phosphorus in those regions is adjusted such that it iswithin the range from 1×10²⁰ to 1×10²¹ atoms/cm³ (typically from 2×10²⁰to 5×10²¹ atoms/cm³).

While this step forms the source regions or drain regions of then-channel type TFTs, a part of the n-type impurity regions 322 through324 formed at the step shown in FIG. 4A is left for the switching TFT.Such residual regions correspond to the LDD regions 15 a through 15 d ofthe switching TFT in FIG. 1.

Next, as shown in FIG. 4C, the resist masks 326 a through 326 c areremoved to form a new resist mask 325. A p-type impurity element (whichis boron in the present embodiment) is added to form impurity regions336 and 337 heavily doped with boron. An ion doping process utilizingdiborane (B₂H₂) is performed here to add boron in a density within therange from 3×10²⁰ to 3×10²¹ atoms/cm³ (typically from 5×10²⁰ to 1×10²¹atoms/cm³).

While the impurity regions 319 and 320 have already been doped withphosphorus in a density within the range from 1×10²⁰ to 1×10²¹atoms/cm³, boron is added here in a density which is at least threetimes the same density. As a result, the previously formed n-typeimpurity regions are completely inverted into the p-type to serve asp-type impurity regions.

Next, as shown in FIG. 4D, a first layer insulation film 338 is formedafter removing the resist mask 325. The first layer insulation film 338may be a single-layer insulation film including silicon or a multi-layerfilm which is a combination of insulation films including silicon. Thethickness of the film may range from 400 nm to 1.5 μm. The presentembodiment employs a structure in which a 800 nm thick silicon oxidefilm is formed on a 200 nm thick silicon oxinitride film.

Thereafter, the n-type or p-type impurity element added in therespective density is activated. The means for annealing may be furnaceannealing, laser annealing or lamp annealing. In the present embodiment,a thermal process at 550° C. is performed for four hours in a nitrogenatmosphere in an electrically heated furnace.

Hydrogenation is further carried out by performing a thermal process ata temperature in the range from 300 to 450° C. for duration in the rangefrom one to twelve hours in an atmosphere including 3 to 100% hydrogen.This is a step to terminate dangling bonds in the semiconductor filmwith thermally excited hydrogen. Plasma hydrogenation (which utilizesplasma-excited hydrogen) may be carried out as another means forhydrogenation.

The hydrogenation process may be included in the formation of the firstlayer insulation film 338. Specifically, the above-describedhydrogenation process may be performed after the 200 nm thick siliconoxinitride film is formed, which is followed by the formation of the 800nm thick silicon.

Next, as shown in FIG. 5A, contact holes are formed in the first layerinsulation film 338 to form source lines 339 through 342, drain lines343 through 345 and a connection line 346. In the present embodiment,these lines are in the form of a multi-layer film having a three-layerstructure provided by continuously forming a 100 nm thick Ti film, a 300nm thick aluminum film including Ti and 150 nm thick Ti film using asputtering process.

Next, a passivation film 347 is formed to a thickness in the range from50 to 500 nm (typically from 200 to 300 nm). In the present embodiment,a silicon oxinitride film having a thickness of 300 nm is used as thepassivation film 347.

At this time, it is advantageous to perform a plasma process utilizing agas including oxygen such as H₂ and NH₃ prior to the formation of thesilicon oxinitride film. Hydrogen excited by this pre-process issupplied to the first layer insulation film 338, and a thermal processis performed to improve the quality of the passivation film 347.Simultaneously, the active layer can be effectively hydrogenated becausethe hydrogen added to the first layer insulation film 338 spreads towardunderlying layers.

Next, as shown in FIG. 5B, a color filter 348 and a fluorescent body 349are formed. Known materials may be used for them. They may be patternedseparately or may alternatively continuously formed and patternedsimultaneously. The thickness of each of them may be chosen within therange from 0.5 to 5 μm (typically from 1 to 2 μm). Especially, theoptimum thickness of the fluorescent body varies depending on thematerial used. Specifically, a too small thickness will reduce colorconversion efficiency, and a too large thickness will result in a largestep and reduce the quantity of light transmitted thereby. Therefore,the optimum thickness must be determined as a tradeoff between thosecharacteristics.

While the present embodiment has referred to the method for colordisplay in which light emitted by the EL layer is subjected to colorconversion, the color filter and fluorescent body may be omitted when amethod is employed in which an EL layer is fabricated in associationwith each of R, G and B.

A second layer insulation film 350 made of organic resin is then formed.Polyimide, polyamide, acrylic resin or BCB (benzocyclobutene) may beused as the organic resin. Especially, acrylic resin having excellentplanarity is preferred because the second layer insulation film isprimarily intended for planarization. In the present embodiment, it isformed of acrylic resin with a thickness that allows any step betweenthe color filter 348 and fluorescent body 349 to be planarized.

Next, a contact hole is formed in the second layer insulation film 350and passivation film 347 down to the drain line 345 to form a pixelelectrode 351. In the present embodiment, a conductive film made of acompound of indium oxide and tin oxide (ITO film) is formed to athickness of 110 nm and is patterned into a pixel electrode. This pixelelectrode serves as the anode of an EL element.

Next, as shown in FIG. 5C, an EL layer 352, a cathode (MgAg electrode)353 and a protective electrode 354 are continuously formed withoutexposing them to the atmosphere. Any known material may be used for theEL layer 352. Known materials include organic materials, and it ispreferable to use an organic material when the driving voltage is takeninto consideration. In the present embodiment, the EL layer isconstituted by a four-layer structure formed by a hole injection layer,a hole transport layer, an emission layer and an electron injectionlayer. While an MgAg electrode is used as the cathode of an EL elementin the present embodiment, any other known material may be used.

The protective film 354 is provided to prevent deterioration of the MgAgelectrode 353 and is preferably constituted by an aluminum film (aconductive film including aluminum). Any other material may obviously beused. Since the EL layer 352 and MgAg electrode 353 are vulnerable tomoisture, continuous formation is preferably extended to the protectiveelectrode 354 without exposing them to the atmosphere in order toprotect the EL layer from the atmosphere.

The thickness of the EL layer 352 may be in the range from 800 to 200 nm(typically from 100 to 120 nm), and the thickness of the MgAg electrodemay be in the range from 180 to 300 nm (typically from 200 to 250 nm).

This completes an active matrix EL display having a structure as shownin FIG. 5C. In practice, it is further preferable to package the displaywith a highly hermetic protective film (a laminate film or the like) toprevent it from being exposed to the atmosphere. In doing so, thereliability of the EL layer is improved by introducing an inertatmosphere into the protective film.

After performing the packaging process to improve hermetic properties, aconnector (flexible printed circuit: FPC) for connecting a terminalcoming from the elements or circuits formed on the substrate and anexternal signal terminal is attached to compete the display as aproduct. An EL display in such a state is referred to as “EL module” inthe present specification.

The active matrix EL display of the present embodiment has very highreliability and can exhibit improved operating characteristics becauseTFTs with optimum structures are provided in the driving circuitportions and the pixel portion.

A TFT having a structure to reduce hot carrier injection is used as ann-channel type TFT 205 of a CMOS circuit forming a part of a drivingcircuit. Driving circuits in this context include shift registers,buffers, level shifters, sampling circuits (sample-and-hold circuits)and the like. D-A converters or latches are further included whendigital driving is performed.

In the present embodiment, as shown in FIG. 5C, the active layer of then-channel type 205 includes a source region 355, a drain region 356, anLDD region 357 and a channel forming region 358, and the LDD region 357overlaps the gate electrode 314 with the gate insulation film 312interposed.

The LDD region is formed only on the side of the drain region in orderto avoid any reduction of the operating speed. In the case of then-channel type TFT 205, the operating speed is of greater importance andthe off-current is a not so serious concern. Therefore, the LDD region357 is preferably overlapped with the gate electrode completely tominimize resistive components. That is, the so-called offset ispreferably eliminated.

There is no particular need for providing an LDD region in the p-channeltype TFT 206 in the CMOS circuit for which there is substantially noconcern about deterioration attributable to hot carrier injection. Theactive layer therefore includes a source region 359, a drain region 360and a channel forming region 361. Obviously, an LDD region may beprovided just as in the n-channel type TFT 205 to cope with hotcarriers.

A sampling circuit is somewhat different from other driving circuits inthat there is a bidirectional flow of a high current through the channelforming region. That is, the functions of the source and drain regionsare switched. Further, there is a need for minimizing the off-currentand, for this reason, it is therefore preferable to provide it with aTFT having function that is intermediate between those of switching andcurrent control TFTs.

Therefore, an n-channel type TFT to form a sampling circuit preferablyhas a structure as shown in FIG. 8. As shown in FIG. 8, a part of LDDregions 71 a and 71 b overlap a gate electrode 73 with a gate insulationfilm 72 interposed. This results in the effect as described above withreference to the current control TFT 202, and the structure for asampling circuit is different only in that a channel forming region 74is sandwiched.

Pixels having a structure as shown in FIG. 1 are provided to form apixel portion. The structures of switching and current control TFTsformed in a pixel will not be described here because they have alreadybeen described with reference to FIG. 1.

Embodiment 2

The present embodiment will refer to a case in which a pixel portion ofan active matrix EL display has a structure different from that shown inFIG. 1.

FIG. 6A shows an example of a structure of a switching TFT differentfrom that shown in FIG. 1. A current control TFT 202, storage capacitor203 and EL element 204 shown in FIG. 6A will not be described becausethey have completely the same structures as those in the firstembodiment. The switching TFT is given new reference numbers only inparts where it is necessary, and the description for FIG. 1 will be usedas it is for the remaining parts.

The switching TFT 201 shown in FIG. 1 and the switching TFT 207 shown inFIG. 6A are different in the positions where the LDD regions are formed.While the LDD regions 15 a through 15 d in FIG. 1 are formed such thatthey do not overlap the gate electrodes 19 a and 19 b, the LDD regionsof the present embodiment are formed such that they partially overlapgate electrodes.

Specifically, as shown in FIG. 6A, a part of LDD regions 50 a through 50d of the switching TFT 207 overlaps gate electrodes 51 a and 51 b with agate insulation film interposed. In other words, the LDD regions 50 athrough 50 d have areas which overlap the gate electrodes 51 a and 51 bwith a gate insulation film interposed.

This makes it possible to minimize the off-current and to preventdeterioration attributable to hot carrier injection. Since a parasiticcapacitance is generated between the gate electrodes and LDD regions,the operating speed may be somewhat lower than that of the structure inFIG. 1. However, a switching TFT with high reliability can be formed ifattention is paid during designing.

FIG. 6B shows an example of a structure of a current control TFTdifferent from that shown in FIG. 1. A switching TFT 201, storagecapacitor 203 and EL element 204 shown in FIG. 6B will not be describedbecause they have completely the same structures as those in the firstembodiment. The current control TFT is given new reference numbers onlyin parts where it is necessary, and the description for FIG. 1 will beused as it is for the remaining parts.

The current control TFT 202 shown in FIG. 1 and the current control TFT208 shown in FIG. 6B are different in the position where the LDD regionis formed. While the LDD regions 33 in FIG. 1 is formed such that itpartially overlaps the gate electrode 35, the LDD region of the presentembodiment is formed such that it partially overlaps a gate electrode.

Specifically, as shown in FIG. 6B, an LDD region 52 of the currentcontrol TFT 208 completely overlaps a gate electrode 53 with a gateinsulation film interposed. In other words, the LDD region 52 does nothave any area which does not overlap the gate electrode 53.

When the lowest voltage of a video (image) signal is applied to the gateof the current control TFT, the EL element emits light if theoff-current is not sufficiently low, which results in a reduction ofcontrast. In the structure in FIG. 1, an LDD region which does notoverlap the gate electrode is provided in order to reduce theoff-current at that time.

However, since the LDD region which does not overlap the gate electrodeacts as a resistive component, some reduction of the operating speed andon-current occurs. Therefore, the structure of the present embodimentwherein such a region is not provided makes it possible to eliminatesuch a resistive component, which allows a higher current to flow. Inthis case, however, a TFT must be used which exhibits a sufficiently lowoff-current when the lowest voltage of a video (image) signal is appliedto the gate of the current control TFT.

The switching 207 in FIG. 6A and the current control TFT in FIG. 6B maybe used in combination. The first embodiment 1 may be referred to forsteps for fabricating them.

Embodiment 3

FIG. 7 shows an example of a pixel configuration according to thepresent embodiment which is different from that shown in FIG. 2B.

In the present embodiment, two pixels as shown in FIG. 2B are providedsuch that they are symmetric about a current source line 212 forsupplying a ground potential. Specifically, as shown in FIG. 7, thecurrent supply line 212 is shard by the two pixels adjacent thereto,which reduces the number of lines required. The structures of the TFTsprovided in the pixel and the like may be kept unchanged.

Such a configuration makes it possible to fabricate a pixel portionhaving higher definition, thereby improving image quality. Theconfiguration according to the present embodiment can be easilyimplemented according to the fabrication steps of the first embodiment,and the TFT structure may be combined with those in the secondembodiment.

Embodiment 4

FIG. 9 shows an example of the formation of a pixel portion having astructure different from that in FIG. 1 according to the presentembodiment. Steps up to the formation of a second layer insulation film44 are in accordance with the first embodiment. A switching TFT 201, acurrent control TFT 202 and a storage capacitor 203 covered by thesecond layer insulation film 44 will not be described because they havethe same structures as those in FIG. 1.

In the present embodiment, a pixel electrode 60, a cathode 61 and an ELlayer 62 are formed after forming a contact hole in the second layerinsulation film 44. They may be provided by continuously formingrespective materials without exposing them to the atmosphere and bypatterning them through simultaneous etching.

In the present embodiment, a 150 nm thick aluminum alloy film (analuminum film including 1 wt % titanium) is provided as the pixelelectrode 60. While any material may be used as the material for thepixel electrode as long as it is a metal material, a material havinghigh reflectivity is preferred.

A 230 nm thick MgAg electrode is used as the cathode 61, and the ELlayer 62 has a thickness of 120 nm. The material described in the firstembodiment may be used to form the EL layer 62.

An insulation film including silicon is then formed to a thickness inthe range from 200 to 500 nm (typically from 250 to 300 nm) and ispatterned to form a protective film 63 having an opening. An anode 64constituted by a transparent conductive film (which is an ITO film inthe present embodiment) is formed thereon to a thickness of 110 nm.Alternatively, the transparent conductive film may be made of a compoundof indium oxide and zinc oxide, tin oxide, indium oxide or zinc oxide.They may be also used with gallium added thereto.

Further, a fluorescent body 65 and a color filter 66 are formed on theanode 64 to complete a pixel portion as shown in FIG. 9.

Red, green or blue light generated by the structure according to thepresent embodiment is emitted oppositely to the substrate on which theTFTs are formed. It is therefore possible to use the substantiallyentire area of a pixel including the area where the TFTs are formed as alight emitting region. This significantly increases the effectivelight-emitting area of the pixel and improves the brightness andcontrast of images.

The configuration according to the present embodiment may be used in anarbitrary combination with either of the configurations according to thesecond and third embodiments.

Embodiment 5

While the first embodiment utilizes laser crystallization as means forforming the crystalline silicon film 302, the present embodiment refersto a case wherein different means for crystallization is used.

In the present embodiment, after an amorphous silicon film is formed, itis crystallized using the technique disclosed in Japanese Laid-Openpatent publication No. 7-130652. The same publication discloses atechnique in which nickel is used as a catalytic element for promotingcrystallization to provide a crystalline silicon film having highcrystallinity.

A step of removing the catalytic element used for crystallization may beperformed when the crystallization step is terminated. In this case, thetechnique disclosed in Japanese Laid-Open patent publication No.10-270363 or 8-330602 may be used to getter the catalytic element.

The TFTs may be formed using the technique disclosed in thespecification of Japanese patent application No. 11-076967 made by theapplicant. The specification of Japanese patent application No.11-076967 may be referred to up to the formation of TFTs, although itdescribe a storage capacitor different from that in FIG. 1.

The principle of the invention is to provide TFTs having appropriatestructures depending on the functional requirements of elements asdescribed in the first embodiment with reference to FIG. 1, but theinvention is not limited to the described method for fabrication.Specifically, the fabrication steps described in the first embodimentare merely an example, and other fabrication steps may be used withoutany problem as long as they can provide the structure in FIG. 1 or FIG.5C according to the first embodiment.

When the structure in FIG. 1 or FIG. 5C is combined with the structureaccording to any of the second through fourth embodiments, thefabrication steps described in the present embodiment may be combinedwith the fabrication of such a structure.

Embodiment 6

A step of etching the gate insulation film 312 may be added between thesteps shown in FIGS. 4A and 4B according to the first embodiment.Specifically, the gate insulation film 312 is etched in a self-aligningmanner using the gate electrodes 313 through 317 and the capacitorforming electrode 318 as masks after an n-type impurity element is addedas shown in FIG. 4A. This etching is continued until the active layer isexposed.

In the present embodiment, dry etching is performed using CHF₃ gas asthe etching gas because the gate insulation film used in the firstembodiment is a silicon oxinitride film. Obviously, there is nolimitation on other etching conditions.

A step of doping the exposed active layer with an n-type impurityelement is then performed as shown in FIG. 4B. The process at this stepcan be performed in a very short time because phosphorus is directlyadded to the active layer without intervention of the gate insulationfilm. Further, since the low acceleration speed during doping can below, damage to the active layer can be reduced.

Thereafter, the steps in the first embodiment may be followed tocomplete an EL display. The configuration according to the presentembodiment may be implemented in an arbitrary combination with thecombination according to any of the first through fifth embodiments.

Embodiment 7

The present embodiment will refer to an active matrix EL display inwhich pixels having a structure different from that in the firstembodiment are formed.

FIG. 10A shows the EL display according to the present embodiment inwhich the TFT structures are the same as those in the first embodiment(see FIG. 5C). According to the present embodiment, a pixel electrode1001, a cathode 1002, an EL layer 1003 and an anode 1004 are formed, andan EL element 1000 is formed by the cathode 1002, EL layer 1003 andanode 1004. At this time, any known conductive film may be used as thepixel electrode 1001. In the present embodiment, an MgAg film is used asthe cathode 1002, and a transparent conductive film obtained by addinggallium oxide to zinc oxide is used as the anode 1004. The EL layer 1003may be formed by combining known materials.

The present embodiment is characterized in that a recess formed in acontact portion of the pixel electrode 1001 (a portion where the pixelelectrode 1001 and the current control TFT 202 are connected) is filledwith an insulator 1005 and in that edges of the pixel electrode 1001 arecovered with an insulator 1006.

The insulator 1005 prevents any defect of the coating of the EL layerattributable to a step by filling the recess. When a contact hole formedin the second layer insulation film 350 is deep (which results in alarge step), defects can occur in the coating of the EL layer to causeshorting between the cathode 1002 and anode 1004. The present embodimentis characterized in that the recess is filled with the insulator 1005 toprevent any defects in the coating of the EL layer.

Further, since a step is similarly formed at the edges of the pixelelectrode 1001 in a size corresponding to the thickness of the pixelelectrode 1001, the insulator 1006 is formed for the same reason as forthe insulator 1005. This makes it possible to reliably prevent shortingbetween the cathode 1002 and anode 1004 at the edges of the pixelelectrode 1001. Another purpose of the insulator 1006 is to preventconcentration of electrical fields in the EL layer 1003 becauseconcentration of electrical fields is likely to occur at the edges ofthe pixel electrode 1001 to promote deterioration of the EL layer 1003.

FIG. 10B shows an example of a structure in which no LDD region isformed in the active layer of a current control TFT. Such a structure ispossible because there is substantially no concern about deteriorationattributable to hot carrier injection when the voltage applied to the ELelement falls to 10 V or less or, more preferably, to 5V or less. In thestructure shown in FIG. 10B, the active layer of the current control TFTis formed by a source region 1010, a drain region 1011 and a channelforming region 1012.

The configuration according to the present embodiment may be used in anarbitrary combination with the configuration according to any of thefirst through sixth embodiments.

Embodiment 8

The driving of an EL display according to the invention can be carriedout on an analog basis using an analog signal as an image signal or on adigital basis using a digital signal.

In the case of analog driving, an analog signal is transmitted to thesource line of the switching TFT, and the analog signal including grayscale information constitutes the gate voltage of the current controlTFT. The current control TFT controls the current that flows through theEL element to control the intensity of the emission of the EL element,thereby allowing gray scale display.

In the case of digital driving, gray scale display referred to as“time-division driving” is performed unlike the gray scale display on ananalog basis. Specifically, the emitting time is adjusted to providevisual appearance that seems like changes in color gradation.

An EL element can be driven at a high speed because it has a responsespeed which is much higher than that of a liquid crystal element.Therefore, it can be regarded as an element suitable for time-divisiondriving in which a single frame is divided into a plurality of subframesto allow gray scale display.

Any driving method may be used because the present invention is atechnique relating to element structures as described above.

Embodiment 9

While an organic EL material is preferably used for the EL layer in thefirst embodiment, the present invention may be implemented using aninorganic EL material. However, since driving voltages for currentlyavailable inorganic EL materials are very high, TFTs having voltagewithstand characteristics that accommodate such driving voltages must beused in the case of analog driving.

It will be possible to apply the present invention to inorganic ELmaterials driven at lower voltages which will possibly be developed inthe future.

The configuration according to the present embodiment may be freelycombined with the configuration according to any of the first throughseventh embodiments.

Embodiment 10

The external view of an EL display device of the present invention isdescribed. Note that FIG. 11A is a top view of the EL display device ofthe present invention, and FIG. 11B is a cross sectional view thereof.

In FIG. 11A, reference numeral 4001 is a substrate, 4002 is a pixelsection, 4003 is a source side driver circuit, and 4004 is a gate sidedriver circuit; each driver circuit reaches to a FPC 4006 through wiring4005, and then connected to the external machines.

A first sealing material 4101, a cover material 4102, fillings 4103 anda second sealing material 4104 are disposed here so as to cover thepixel section 4002, source side driver circuit 4003 and gate side drivercircuit 4004.

FIG. 11B corresponds to a cross section at line A-A′ of FIG. 11A, and adriver circuit 4201 (note that an n-channel TFT and a p-channel TFT isformed here) which comprises the source side driver circuit 4003 and acurrent controlling TFT 4202 which comprises the pixel section 4002 areformed over a substrate 4001.

A TFT having the same structure as the n-channel TFT 205 and thep-channel TFT 206 of FIG. 5C is used for the driver TFT 4201, and a TFThaving the same structure as the n-channel TFT 202 of FIG. 1 is used forthe current controlling TFT 4202 in the present embodiment. Further, astorage capacitor (not shown in the Figure) connected to the gate of thecurrent controlling TFT 4202 is provided in the pixel section 4002.

An interlayer insulating film (flattening film) 4301 comprising a resinmaterial is formed over the driver TFT 4201 and the pixel TFT 4202, anda pixel electrode (cathode) 4302 is formed thereon which electricallyconnects to the drain of pixel TFT 4202. A conductive film having asmall work function is used for the pixel electrode 4302. A conductivefilm comprising an element that belongs to group 1 or 2 of periodictable (typically a conductive film of aluminum, copper or silver thatincludes alkali metal element or alkaline earth metal) can be used.

An insulating film 4303 is formed over the pixel electrode 4302, and anopening section is formed in the insulating film 4303 on the pixelelectrode 4302. An EL (electro-luminescence) layer 4304 is formed overthe pixel electrode 4302 at this opening section. A publicly knownorganic EL material or inorganic EL material can be used for the ELlayer 4304. Further, though there are small molecular type (monomertype) material and polymer material among the organic EL materials,either may be used.

A technique of public domain such as evaporation technique or coatingtechnique may be utilized for the manufacturing method of the EL layer4304. The structure of the EL layer may be a laminate structure or asingle layer structure, in which hole injection layer, hole transportlayer, light emitting layer, electron transport layer or electroninjection layer are freely combined.

An anode 4305 comprising a transparent conductive film is formed overthe EL layer 4304. A compound of indium oxide and tin oxide, a compoundof indium oxide and zinc oxide, indium oxide, tin oxide, zinc oxide or acompound added with gallium in these compounds, can be used for thetransparent conductive film.

It is preferable to remove as much as possible of the moisture andoxygen existing in the interface between the anode 4305 and the EL layer4304. It is therefore necessary to take measures such as depositing thetwo continuously inside a vacuum, or forming the EL layer 4304 innitrogen or noble gas atmosphere and then forming the anode 4305 withoutexposure to oxygen and the moisture. It is possible to perform the abovefilm deposition in the present embodiment by using a multi-chambersystem (cluster tool system) deposition apparatus.

The anode 4305 is then electrically connected to the wiring 4005 in aregion denoted as 4306. The wiring 4005 is a wiring for applying apreset voltage to the anode 4305, and electrically connected to FPC 4006through an anisotropic conductive film 4307.

An EL element comprising pixel electrode (cathode) 4302, EL layer 4303and anode 4305 is thus formed. This EL element is covered by a firstsealing material 4101 and covering material 4102 which is stuck to thesubstrate 4001 by the first sealing material 4101, and sealed byfillings 4103.

A glass material or a plastic material (including plastic film) can beused for the covering material 4102. FRP (fiberglass-reinforcedplastics) plate, PVF (poly vinyl fluoride) film, Myler film, polyesterfilm or acrylic resin film can be used for the plastic material.

A ultraviolet-ray curing resin or a thermosetting resin can be used forthe fillings 4103, and PVD (poly vinyl chloride), acrylic, polyimide,epoxy resin, silicone resin, PVB (polyvinyl butyral) or EVA (ethylenevinyl acetate) can be used. The degradation of EL elements can beprevented when a drying agent (preferably barium oxide) is provided onthe inside of the fillings 4103.

Further, spacers may be contained in the fillings 4103. In this case itis possible to give moisture absorption property to the spacer itselfwhen the spacers are formed from barium oxide. Further when spacers aredisposed it is effective to provide a resin film over the anode 4305 asa buffer layer which relieves the pressure from the spacers.

Further, wiring 4005 is electrically connected to the FPC 4006 byinterposing anisotropic conductive film 4307. The wiring 4005 transmitsto FPC 4006, signal sent to pixel section 4002, source side drivercircuit 4003 and gate side driver circuit 4004, and is electricallyconnected to an external machine through the FPC 4006.

In the present embodiment a second sealing material 4104 is disposed tocover the exposed portion of the first sealing material 4101 and a partof FPC 4006 which is a structure to thoroughly shut the EL elements fromthe external atmosphere. In this way an EL display device having thecross sectional structure of FIG. 11B is formed. Note that it isacceptable to fabricate the EL display device of the present embodimentby combining any constitution of the Embodiments 1 to 9.

Embodiment 11

The present embodiment shows in FIGS. 12A to 12C an example of pixelstructure of the EL display device of the present invention. Note thatin this embodiment, reference numeral 4601 denotes a source wiring of aswitching TFT 4602; 4603, gate wirings of the switching TFT 4602; 4604,a current controlling TFT; 4605, a capacitor; 4606 and 4608, electriccurrent supply line; and 4607, an EL element.

FIG. 12A shows an example in which the current supply line 4606 isshared by two pixels. In other words, this example is characterized inthat two pixels are formed so as to be axisymmetric with respect to thecurrent supply line 4606. In this case, the number of current supplylines can be reduced, further enhancing the definition of the pixelsection.

FIG. 12B shows an example in which the current supply line 4608 isarranged in parallel with the gate wirings 4603. Though the currentsupply line 4608 is arranged so as not to overlap with the gate wirings4603 in FIG. 12B, the two can overlap with each other through aninsulating film if the lines are formed in different layers. In thiscase, the current supply line 4608 and the gate wirings 4603 can sharetheir occupying area, further enhancing the definition of the pixelsection.

FIG. 12C is characterized in that the current supply line 4608 isarranged, similar to the structure in FIG. 12B, in parallel with thegate wirings 4603 and, further, two pixels are formed to be axisymmetricwith respect to the current supply line 4608. It is also effective toarrange the current supply line 4608 so as to overlap with one of thegate wirings 4603. In this case, the number of current supply lines canbe reduced, further enhancing the definition of the pixel section.

Note that it is possible to freely combine the constitution of thepresent embodiment with any of the constitution of the Embodiments 1 to10.

Embodiment 13

In the present embodiment examples of pixel structures of EL displaydevices are shown in FIGS. 13A and 13B. Note that in the presentembodiment reference numeral 4701 is a source wiring of switching TFT4702; 4703, gate wiring of switching TFT 4702; 4704, current controlTFT; 4705, capacitor (can be omitted); 4706, current supply line; 4707,power source control a TFT; 4708, gate wiring for power source control;and 4709, EL element. Japanese Patent Application No. 11-341272 may bereferred as to operation of power source control 4707.

Further, though the present embodiment provides the power source controlTFT 4707 between current control TFT 4704 and EL element 4708, it may bea structure in which current control TFT 4704 is provided between powersource control TFT 4707 and EL element 4708. Moreover, it is preferableto form the power source control TFT 4707 in the same structure as thecurrent control TFT 4704, or formed connected in series by the sameactive layer.

FIG. 13A is an example of a case in which current supply line 4706 isshared between 2 pixels. Namely it is characterized in that 2 pixels areformed axisymmetric around the current supply line 4706. In this case,because the number of current supply lines can be reduced, pixel sectioncan be further made into high definition.

FIG. 13B is an example of a case in which current supply line 4710 isprovided in parallel with the gate wiring 4703 and power supply controlgate wiring 4711 is provided in parallel with the source wiring 4701.Though the current supply line 4710 and gate wiring 4703 are provided soas not to overlap in FIG. 13B, these can be provided to overlap byinterposing an insulating film if these are wirings formed in differentlayers. In this case, the area used exclusively by the current supplyline 4710 and the gate wiring 4703 can be shared, so the pixel sectioncan be made even higher definition.

Note that it is possible to freely combine the constitutions of thepresent embodiment with any constitution of Embodiments 1 to 10.

Embodiment 14

This embodiment gives a description with reference to FIGS. 14A and 14Bon an example of the pixel structure for the EL display device of thepresent invention. In this embodiment, reference numeral 4801 denotes asource wiring of a switching TFT 4802; 4803, a gate wiring of theswitching TFT 4802; 4804, a current controlling TFT; 4805, a capacitor(can be omitted); 4806, a current supply line; 4807, an erasing TFT;4808, an erasing gate electrode; and 4809, an EL element. JapanesePatent Application No. 11-338786 may be referred for the operation ofthe erasing TFT 4807.

A drain of the erasing TFT 4807 is connected to a gate of the currentcontrolling TFT 4804 so that the gate voltage of current controlling TFT4804 can forcibly be changed. The erasing TFT 4807 may be either of Nchannel type or of P channel type, but preferably has the same structureas the switching TFT 4802 to reduce the OFF current.

FIG. 14A shows an example in which two pixels share the current supplyline 4806. That is, the example is characterized in that two pixels areformed such that they are axisymmetric with respect to the currentsupply line 4806. In this case, the number of current supply lines canbe reduced to obtain even higher definition for the pixel portion.

FIG. 14B shows an example in which a current supply line 4810 is formedin parallel with the gate wiring 4803 and an erasing gate wiring 4811 isformed in parallel with the source wiring 4801. The current supply line4810 and the gate wiring 4803 are formed so as not to overlap with eachother in FIG. 14B. However, they may overlap with each other through aninsulating film as long as the two are wirings formed in differentlayers. In this case, the current supply line 4810 and the gate wiring4803 share their occupied areas to obtain even higher definition for thepixel section.

Note that it is possible to freely combine the constitution of thepresent embodiment with any constitution of Embodiments 1 to 10.

Embodiment 15

An EL display device according to the present invention may have anynumber of TFTs is in a pixel. Though shown in Embodiments 13 and 14 areexamples in each of which 3 TFTs are formed in a pixel, 4 to 6 TFTs maybe provided. The present invention can be carried out without puttinglimitation to the pixel structure of the EL display device.

Note that it is possible to freely combine the constitution of thepresent embodiment with any of the constitution of Embodiments 1 to 10.

Embodiment 16

An EL display devices formed by executing the present invention can beutilized for a display section of various electric machines. Forexample, a display incorporating an EL display device of the presentinvention which has a diagonal 20 to 60 inches may be used for watchingTV broadcasting etc. Note that the display incorporating an EL displaydevice into the body includes all kinds of display for informationdisplay such as a display for personal computer, a display for receivingTV broadcasting, a display for displaying advertisements etc.

Following can be given as other electric machines of the presentinvention: video cameras; digital cameras; goggle type displays (headmounted displays); navigation systems; sound reproduction devices (carstereos, audio components etc.); notebook type personal computers; gamemachines; portable information terminals (mobile computers, portabletelephones, portable game machines or electronic books, etc.); imagereproduction devices (a device which incorporates a display sectiondisplaying an image by reproducing an image recorded in a recordingmedium), etc. Examples of these electric machines are shown in FIGS. 15Ato 16B.

FIG. 15A is a display which incorporates an EL display device into thebody, and comprises a body 2001, supporting arm 2002 and a displaysection 2003. The EL display device of the present invention can be usedfor the display section 2003. Because such display is spontaneous lightemitting type back light is not required and a display section thinnerthan a liquid crystal display can be made.

FIG. 15B is a video camera, and comprises: a main body 2101; displaysection 2102; a voice input section 2103; operation switches 2104; abattery 2105; and an image receiving section 2106. The EL display deviceof the present invention can be used in the display section 2102.

FIG. 15C is a part (right hand side) of head mounted type EL display andcomprises: a main body 2201; signal cable 2202; head mounting band 2203;a display section 2204; optical system 2205; and an EL display device2206. The present invention can be used in the EL display device 2206.

FIG. 15D is an image reproduction device (DVD reproduction device inconcrete) incorporating a recording medium and comprises: a main body2301; a recording medium (DVD etc.) 2302; operation switches 2303; adisplay section (a) 2304; and a display section (b) 2305. The displaysection (a) displays mainly image information, and the display section(b) displays mainly literal information. The EL display device of thepresent invention can be used in these display sections (a) and (b).Note that a home use game machines etc. are included in the imagereproduction device incorporating a recording medium.

FIG. 15E is a mobile computer, and comprises: a main body 2401; a camerasection 2402; an image receiving section 2403; operation switches 2404;and a display section 2405. The EL display device of the presentinvention can be used in the display section 2405.

FIG. 15F is a personal computer and comprises: a main body 2501; a body2502; a display section 2503; and a key board 2504. The EL displaydevice of the present invention can be used in the display section 2503.

Note that the application to front type or rear type projector becomespossible by enlarging and projecting the light comprising output imageinformation by lenses and optical fibers if the luminance of the lightemittance further improves in future.

Since the parts where light is emitted consume electricity in the lightemitting device, it is preferable to display information so as tominimize the light emitting parts as possible. Accordingly, specificallyin case of using a light emitting device in the display section whichmainly displays literal information such as a portable telephone or asound reproduction device, it is preferable to drive so as to form theliteral information by light emitting parts on the background ofnon-light emitting parts.

FIG. 16A is a portable telephone, and comprises: a main body 2601; avoice output section 2602; a voice input section 2603; a display section2604; operation switches 2605; an antenna 2606. The EL display device ofthe present invention can be used in the display section 2604. Note thatthe display section 2604 can reduce electricity consumption of aportable telephone by displaying a white colored letters on a blackcolored background.

FIG. 16B is a sound reproduction device, a car stereo in concrete, andcomprises a main body 2701, a display section 2702, and operationswitches 2703 and 2704. The EL display device of the present inventioncan be used in the display section 2702. Though the present embodimentshows a car stereo for mounting on a vehicle, it may be used for a soundreproduction device of portable type or a home use. Note that thedisplay section 2702 can reduce electricity consumption of a portabletelephone by displaying a white colored letters on a black coloredbackground. This is specifically effective in a portable type soundreproduction device.

As described above, the applicable range of the present invention isvery large, and it is possible to apply to electric machines of variousareas. Further, the electric machines of the present embodiment mayapply any constitution of EL device shown in Embodiments 1 to 15.

Effect of the Invention

By using this invention, it is possible to provide TFTs havingappropriate characteristics in accordance with performance required forelements on the same insulator, and to provide high operatingperformance and reliability of the EL display device.

Concretely, it is possible to use separately a TFT oriented toward highoperation speed and the TFT oriented toward low off-current on the sameinsulator. Accordingly, in the pixel of the EL display device, theswitching TFT can obtain sufficient lower off-current, and the currentcontrol TFT can also obtain the sufficient lower off current bypreventing from deterioration attributable to injection of hot carriers.

Furthermore, by using such an EL display device as a display device, itis possible to produce applied apparatus (electronic apparatus) havinghigh durability (high quality).

What is claimed is:
 1. A display device comprising: a pixel comprising:a first transistor comprising a first region of a semiconductor layer, agate insulating film, and a gate electrode; a second transistorelectrically connected to the first transistor; a storage capacitorcomprising a second region of the semiconductor layer, the gateinsulating film, and a capacitor forming electrode; and an EL elementelectrically connected to the second transistor; an insulating film overthe first transistor, the second transistor, and the storage capacitor;a current supply line electrically connected to the capacitor formingelectrode through a contact hole in the insulating film, and one of asource region and a drain region of the second transistor; and a gateline intersecting with the current supply line, wherein the EL elementis formed over the insulating film, wherein the capacitor formingelectrode and the gate electrode of the first transistor are over and incontact with the gate insulating film of the first transistor, whereinthe capacitor forming electrode forms in parallel to the gate line whileoverlapping the second region of the semiconductor layer, wherein thegate electrode of the first transistor is double gate structure, andwherein the capacitor forming electrode and the double gate electrode ofthe first transistor are formed from a conductive film.
 2. The displaydevice according to claim 1, wherein the first transistor has an LDDregion which does not overlap with the gate electrode of the firsttransistor.
 3. The display device according to claim 1, wherein thesecond transistor has an LDD region which partially overlaps with a gateelectrode of the second transistor.
 4. The display device according toclaim 1, wherein the first transistor has an LDD region which partiallyoverlaps with the gate electrode of the first transistor.
 5. The displaydevice according to claim 1, wherein a gate electrode of the secondtransistor is electrically connected to one of a source region and adrain region of the first transistor.
 6. The display device according toclaim 1, wherein a portion of the current supply line is over thecapacitor forming electrode.
 7. A display device comprising: a pixelcomprising: a first transistor comprising a first region of asemiconductor layer, a gate insulating film, and a gate electrode; asecond transistor electrically connected to the first transistor; astorage capacitor comprising a second region of the semiconductor layer,the gate insulating film, and a capacitor forming electrode; a colorfilter; and an EL element overlapped with the color filter andelectrically connected to the second transistor; an insulating film overthe first transistor, the second transistor, and the storage capacitor;a current supply line electrically connected to the capacitor formingelectrode through a contact hole in the insulating film, and one of asource region and a drain region of the second transistor; and a gateline intersecting with the current supply line, wherein the EL elementis formed over the insulating film, wherein the capacitor formingelectrode and the gate electrode of the first transistor are over and incontact with the gate insulating film of the first transistor, whereinthe capacitor forming electrode forms in parallel to the gate line whileoverlapping the second region of the semiconductor layer, wherein thegate electrode of the first transistor is double gate structure, andwherein the capacitor forming electrode and the double gate electrode ofthe first transistor are formed from a conductive film.
 8. The displaydevice according to claim 7, wherein the first transistor has an LDDregion which does not overlap with the gate electrode of the firsttransistor.
 9. The display device according to claim 7, wherein thesecond transistor has an LDD region which partially overlaps with a gateelectrode of the second transistor.
 10. The display device according toclaim 7, wherein the first transistor has an LDD region which partiallyoverlaps with the gate electrode of the first transistor.
 11. Thedisplay device according to claim 7, wherein a gate electrode of thesecond transistor is electrically connected to one of a source regionand a drain region of the first transistor.
 12. The display deviceaccording to claim 7, wherein a portion of the current supply line isover the capacitor forming electrode.